The present invention relates to a method for selecting an antenna beam of a base station of a radio system from among two or more optional antenna beams. The antenna beams are arranged to receive signals associated with the same logical channel. The antenna means of the base station are provided with measuring means for measuring the signal level of a received signal. The signal level of the signals received by different antenna beams is measured and the antenna beam that has received the signals with the strongest signal strength is selected for use. The signals received by the selected antenna beam are supplied further to a receiver unit of the base station situated at a distance from the antenna means.
The invention also relates to a base station of a radio system whose antenna means include means for receiving signals associated with the same logical channel by at least two antenna beams, measuring means for measuring the signal level of the signals received by the antenna beams, and control means for selecting one or more antenna beams and for supplying the signals received by means of the selected antenna beam or antenna beams further to a receiver unit of the base station arranged at a distance from the antenna means.
The invention further relates to a cellular radio system including a base station comprising antenna means arranged at a distance from the base station for receiving radio signals associated with the same logical channel by at least two antenna beams, measuring means for measuring the signal level of the signals received by the antenna beams, and control means for selecting one or more antenna beams and for supplying the signals received by means of the selected antenna beam or antenna beams further to a receiver unit of the base station, and a network management center connected by means of a data transmission connection to the base station.
This invention especially relates to the size of the radio coverage area of base stations in a cellular radio system. It has proved to be very difficult to cover large, almost uninhabited areas by a cellular radio system as the need for traffic capacity in these areas is generally so low that increasing the coverage area. of the system by new base stations is an alternative that is too expensive. To cover a large area by a few base stations is often almost impossible because of path attenuation. Especially on higher frequencies, such as 1800 or 1900 MHz which has a significant, path attenuation.
Most often the geographical area covered by the base station can be enlarged at least to some extent by increasing the transmission power of base station transmitters, but a problem will then be the insufficient transmission power of subscriber terminal equipment which in practice determines the largest possible size of the coverage area. That is, although the subscriber terminal equipment were able to receive signals sent by the base station on a higher power than normal, the base station is not able to receive signals sent by subscriber terminal equipment.
One known solution for improving the reception ability of base stations is that narrowing of antenna beams of base stations narrowed, whereby a geographical area of a specific size is no longer attempted to be covered by one wide beam antenna element, but several narrow beam receiving antenna elements are directed thereto. The division of the geographical area covered by the base station into narrower beams than before entails, however, that the number of beams and the available antenna elements will grow significantly, which in turn sets new requirements for the cabling of the base station, for example. As a base station is generally situated at a distance from antenna means, that is, in practice on a ground level next to an antenna mast, it is advantageous to the cabling of the base station that signals received by only one or two (diversity reception) antenna beams, for example, be directed to the base station because the number of cables needed between the antenna mast and the base station remains low.
A base station is previously known where an antenna mast is provided with a separate control unit, that is, a so-called RSSI receiver (Received Signal Strength Indication). The antenna mast comprises means for receiving signals with several narrow beam antenna beams, in which case the signals received by different antenna beams and associated with the same logical channel are directed to the inputs of the control unit. The control unit then measures the signal level of a signal received for each signal supplied thereto and selects the signal with the strongest signal strength to be supplied further to the receiver unit of the base station.
The most significant weakness of the prior art base station explained above is that because the control unit selects the signal to be supplied to the base, station. only on the basis of its signal strength, the control unit may under unfavorable conditions be locked into an incorrect signal. That is, if in the environment of the base station, there is some strong source of disturbance (e.g. a faulty radio link) whose frequency corresponds to the frequency channel used by a mobile station, for example, and whose signals are heard more strongerly at the base station than the signals of the mobile station, it may happen that the control unit transmits to the receiver unit of the base station a signal originating from the source of disturbance in place of the signal sent by the mobile station, in which case the connection between the base station and the mobile station will. break.
The object of the present invention is to solve the problem explained above and attain a method for selecting an antenna beam of a base station in such a manner that the best possible signal will be definitely transmitted to the base station, even under disturbed conditions. This object is achieved with the method of the invention that is characterized by monitoring the quality of the signals received by the receiver unit of the base station by means of said selected antenna beam, and selecting some other antenna beam for use in place of the first selected antenna beam if the quality of the signals received by the receiver unit falls below a predetermined reference level.
The invention also relates to a base station to which the method of the invention can be applied. The base station comprises means for measuring the quality of the signals received by the receiver unit from the antenna means and for generating a quality signal responsive to the quality of the signals and for supplying it to the control means. The control means are arranged to select one or more antenna beams on the basis of the signal level measured by the measuring means and the quality signal supplied to the control means.
The invention is based on the idea that the selection of the most suitable antenna beam will be significantly easier, and that the reception conditions of the base station are significantly improved especially in disturbed conditions when in addition to the strength of the received signal, the quality of the signal is taken into consideration in the,selection of the antenna beam. As known base stations are already able to determine the quality of the signals received by their receiver units, this feature can be utilized according to the invention so that a signal describing the quality of the signal is directed from the base station to a control unit arranged in association with the antenna of the base station. In that case the control unit is able to pay attention both to the signal strength and signal quality in the selection of an antenna beam. As the information representing the quality of the received signal is conveyed to the control unit from the base station, the method of the invention can be employed without requiring complicated features that raise the price of the control unit. It is therefore necessary to add only a port to a control unit (RSSI receiver) known per se for receiving a signal representing quality and logic (a computer program) for selecting an antenna beam in accordance with the invention.
The invention further relates to a cellular radio system where the 10 method of invention and the base station of the invention can be employed. In the cellular radio system the base station comprises means for measuring the quality of the signals received by the receiver unit from the antenna means and for generating a quality signal responsive to the quality of the signals and for supplying it to the control means, in which case the control means are arranged to select one or more antenna beams on the basis of the signal level measured by the measuring means and the quality signal supplied to the control means. The base station further comprises means for sending information representing the signal level of the received signals and the quality of the signals received by the receiver unit separately for each antenna beam to the network management center by means of,said data transmission connection, and that the network management center includes data; processing means for processing data received from the base station to determine the disturbance level in the environment of the base station.
By means of values representing the strength and quality of the signals received by the base station, the operator may collect, for the network management center information about disturbances in the geographical area covered by the cellular radio system. That is, because the base station is able to identify a disturbed signal in such a manner that the strength of the signal is great but the quality of the signal falls below the predetermined level (as it is an incorrect signal), the base station is able to collect a significant amount of information that the operator can utilize for scanning disturbances. This information is for example the directions in which the source of disturbance is situated, the frequency of disturbance and the time when the disturbance occurred. By means of this information, the operator may try to trace the source of disturbance, especially if it is a regularly occurring disturbance.
The preferred embodiments of the method and the base station of the invention appear from the appended dependent claims.